Understanding the mechanism of inheritance of chromosomes in human cells is a fundamental problem in biology and has direct relevance to cancer research. Alterations to the normal pattern of inheritance promote progression to cancer by the accumulation of somatic genetic aberrations in the genome of tumor cells. The regulatory pathways investigated by others in this Program Project directly control the DNA replication and mitosis. The proposed studies focus how a cell duplicates and segregates chromosomes and are based on the observation that Origin Recognition Complex (ORC) in human cells participates in many of these processes. ORC is required for the initiation of DNA replication and recent research has demonstrated that ORC, or ORC subunits participate in other aspects of chromosome inheritance and cell division. The first aim will address the dynamic assembly of ORC during the cell division cycle and determine how this process is regulated. A second aim will identify and characterize ORC associated proteins in chromosome inheritance, suing both proteomic methods (with Core D) and genetic approaches with an RNAi library (Core B) to discover cellular proteins essential for Epstein Barr Virus replication. A third aim will expand research on the role of ORC in maintenance of heterochromatin and a potential relationship with the RNAi machinery maintaining heterochromatin. A final aim will study the role of ORC subunits in centrosomes duplication and control chromosome segregation. Although the cell division cycle regulatory machinery coordinates the multiple stages of the chromosome inheritance cycle, research in this Project suggests a more fundamental connection between the actual processes of DNA replication and chromosome segregation. Thus these studies on chromosome inheritance are directly relevant to the overall goals of the program project and the other projects in the Program.